Difference.

train cpu 825He will be watching TV at 9:00 a.m.

A: What is he doing tomorrow morning at 9?

B: He is /will be watching TV.

train cpu 825He will have been watching TV since 9 a.m.

This sentence needs context to make sense. For future perfect, you need a future time reference.

A: Why won't he come to see the movie with us tomorrow night? (which is a Friday)
B: He will have been watching TV since 9 am, so by then he will be fed up with seeing any movies.

But I don't think this structure with 'since' is very frequently used by native speakers.

train cpu 8251) He will be watching TV at 9:00 a.m.
2) He will have been watching TV since 9 a.m.

1) He will be watching TV at 9:00 a.m.

- - 7am - - - - - 8am - - - - - 9am - - - - - 10am - -
... breakfast ... reading .... watching TV ... shopping .
............................................................^.....................................

2) (At 11am) He will have been watching TV since 9 a.m.

- - 7am - - - - - 8am - - - - - 9am - - - - 10am - - - 11am
... breakfast ... reading ........... [ - - - - - watching TV - - - >
..........................................................[ .............................................^....

CJ